How Reliable is Laboratory Testing?

Four indicators are most commonly used to determine the reliability of a clinical laboratory test. Two of these, accuracy and precision, reflect how well the test method performs day to day in a laboratory. The other two, sensitivity and specificity, deal with how well the test is able to distinguish disease from absence of disease.

The accuracy and precision of each test method are established and are frequently monitored by the professional laboratory personnel. Sensitivity and specificity data are determined by research studies and are generally found in medical literature. Although each test has its own performance measures and appropriate uses, laboratory tests are designed to be as precise, accurate, specific, and sensitive as possible. These basic concepts are the cornerstones of reliability of your test results and provide the confidence your healthcare provider has in using the clinical laboratory.

Accuracy and Precision

Statistical measurements of accuracy and precision reveal a lab test’s basic reliability. These terms, which describe sources of variability, are not interchangeable. A test method can be precise (reliable reproducibility) without being accurate (measuring what it is supposed to measure and its true value) or vice versa.

Accuracy (Trueness)
A test method is said to be accurate when the test value approaches the absolute “true” value of the substance (analyte) being measured. Results from every test performed are compared to known “control specimens” that have undergone multiple evaluations and compared to the “gold” standard for that assay, thus analyzed to the best testing standards available.

Precision (Repeatability)
A test method is said to be precise when repeated analyses on the same sample give similar results. When a test method is precise, the amount of random variation is small. The test method can be trusted because results are reliably reproduced time after time.

Although a test that is 100% accurate and 100% precise is ideal, in practice, test methodology, instrumentation, and laboratory operations all contribute to small but measurable variations in results. The small amount of variability that typically occurs does not usually detract from the test’s value and statistically is insignificant. The level of precision and accuracy that can be obtained is specific to each test method but is constantly monitored for reliability through comprehensive quality control and quality assurance procedures. Therefore, when your blood is tested more than once by the same laboratory, your test results should not change much unless your condition has changed. There may be some differences between laboratories in precision and accuracy due to different analytical instrumentation or methodologies, however, the test results are reported with standardized reference intervals specific for that laboratory. This helps your healthcare provider to correctly interpret the information and its relevance to that reference interval.

Sensitivity and Specificity

The tests that a provider chooses in order to diagnose or monitor a medical condition are based on their inherent ability to distinguish whether you have the condition or do not have the condition. Depending on the symptoms and medical history, a provider will order tests to confirm a condition (tests with high sensitivity) or tests to rule out the condition (tests with high specificity).

Sensitivity
Sensitivity is the ability of a test to correctly identify individuals who have a given disease or condition. For example, a certain test may have proven to be 90% sensitive. If 100 people are known to have a certain disease, the test that identifies that disease will correctly do so for 90 of those 100 cases (90%). The other 10 people (10%) tested will not show the expected result for this test. For that 10%, the finding of a “normal” result can be misleading and is termed false-negative. The more sensitive a test, the fewer false-negative results will be produced.

Specificity
Specificity is the ability of a test to correctly exclude individuals who do not have a given disease or condition. For example, a certain test may have proven to be 90% specific. If 100 healthy individuals are tested with that method, only 90 of those 100 healthy people (90%) will be found “normal” (disease-free) by the test. The other 10 people (who do not have the disease) will appear to be positive for that test. For that 10%, their “abnormal” findings are a misleading false-positive result. When it is necessary to confirm a diagnosis that requires dangerous therapy, a test’s specificity is one of the crucial indicators. A patient who has been told that he or she is positive for a specific test yet truly does not have that disease may be subjected to potentially painful or dangerous treatment, additional expense, and unwarranted anxiety, and may be required to be retested. The more specific a test, the fewer false-positive results it produces.

The FDA requires that developers and manufacturers of a new test provide target values for test results and provide evidence for the expected ranges as well as information on test limitations and other factors that could generate false results. Thus it is critical for the healthcare provider to correlate the laboratory results with an individual’s clinical condition to determine if repeat testing would be needed.

Laboratory testing is subject to many factors that potentially could adversely affect the integrity of the sample and prevent the timely reporting of an accurate test result to your provider. Typically these are grouped into three areas that track the sample from beginning to end of the process.

• Pre-Analytical is the term used to describe things that happen from the time the test is ordered to the time the sample arrives in the lab.
• Analytical is the term used to describe the things that happen during the handling and analysis of the sample in the laboratory.
• Post-Analytical is the term used to describe what happens after a result is obtained and includes how and when it is reported to your provider.

Pre-Analytic Activities
From the time a test is ordered, a chain of events is set into motion. All these steps must be done the right way to ensure that an acceptable sample arrives at the lab. The following are some of the areas where errors can occur, even though standard procedures are in place to prevent such errors.

• Test ordering process – the provider must order the correct test using the correct test name or code.
• Patient preparation for the test – the patient should have received appropriate instructions about diet, fasting, medications, etc. to ensure that the sample will not contain substances that interfere with the test.
• Patient identification – the person collecting the sample needs to confirm that the person is indeed the patient on whom the test was ordered. In hospitals, this means checking for a wristband and confirming proper identification of the patient. When you are asked to give your name and some other identifying information, it is to ensure that you and your sample are correctly matched. If this isn’t done when a sample is taken, make sure you check to see that your sample is identified correctly.
• Completeness of patient information – the sample must be carefully labeled with two identifiers (usually patient name and date of birth or medical record number and the date of service) to ensure that the sample is associated with the correct patient. A hard copy requisition or electronic version must accompany the specimen and includes the tests ordered, patient’s demographics, ordering practitioner’s demographics, date of service, and the phlebotomist’s identification.
• Specimen collection procedures – the sample must be collected in the correct type of container and mixed with the right preservative, when appropriate. The laboratory staff who collect samples receive special training on how to collect samples for each kind of test.
• Transport to the lab – some samples need to be kept cold, while some need to be processed and tested within a limited time, so careful handling and prompt transportation are important components to sample integrity. In addition, ambient temperatures during transport, with extremely hot or cold weather, may also have an impact on specimen integrity and must be addressed accordingly.

Automated ordering systems are increasingly used to expedite orders and minimize errors in sample collection and test requests. Use of computer technology (Laboratory Information System or LIS) allows healthcare providers to quickly input the test orders themselves. Since there are thousands of laboratory tests that may be selected from a laboratory’s test menu, this process minimizes the chance for misunderstanding of what tests are desired. Many of these systems can generate labels that include a bar code for tracking and identification of a patient and the specific tests ordered. It can also provide supplemental information on the sample size needed (how much blood needs to be drawn), types of containers or blood tubes to be used to collect the sample, and information about handling, transporting, and storing samples.

For some tests, your diet, medications, and exercise history are important for proper interpretation of results. In some cases, special preparation must be taken to get reliable results for a test (such as fasting overnight before blood glucose or cholesterol is checked). A patient who does not follow preparation instructions or provides insufficient information to the healthcare provider undermines the entire quality assurance effort for a specific test.

Analytic Activities
Many test methods use automated analyzers, thus minimizing opportunities for human error. In the past decade or so, integrated computer technology has greatly enhanced the ability to accurately and consistently manage proper specimen identification, process of specimen testing, and test result reporting. In addition, these complex instruments can incorporate surveillance systems to detect malfunctions or other discrepancies and bring them to the attention of the laboratory staff.

Once the specimen is received in the laboratory, quality assurance procedures guide and monitor all related activities and include the following:

• Instrument operations – all instruments are calibrated and tested regularly according to standard protocols that address quality control, maintenance, and operation.
• Valid test reagents – all test materials have an expiration date. The lab has routine procedures to make sure that test materials are good and have not gone past their expiration date.
• Testing personnel – laboratory personnel who perform the testing must meet certain educational and training criteria. Many are professionally certified and in some states are required to be licensed in order to perform laboratory testing. (See Who’s Who in the Lab.)

Post-Analytic Activities
After the test is completed, the result must be delivered in a timely fashion to the healthcare provider.

• Report sent to appropriate party – the report needs to be sent to the provider who ordered the test and any other healthcare professional identified to receive the report in order to diagnose, treat, or manage their patient.
• Timely reporting of data – for most laboratory tests, an expected turn-around-time (TAT) is established and is often monitored as part of the quality assurance program.
• Reference intervals – laboratory reports must include a framework for interpreting the results, which is referred to as the “reference interval.” Each laboratory must establish a reference interval for every test they offer or provide a written interpretation of the test results. In general, small variations in these intervals may be observed from laboratory to laboratory due to differences in instrumentation or testing methods. They also may be adjusted for biological factors such as sex, age, or other clinical situations. When a specific test is used to monitor a patient over a period of time, using the same laboratory to obtain those results provides a level of consistency to the treatment plan.
• “Critical limits” – each laboratory has in place a system for immediate reporting to the ordering provider any finding that reflects a critical or life-threatening condition. When a test value falls within the critical limits, immediate action is required. Failure to report critical values may result in a patient’s death or a medical condition that cannot be reversed.

Most laboratory results are collated and managed by a sophisticated computer system (Laboratory Information System or LIS) capable of sending electronic reports to the healthcare provider by directly printing the report in a provider’s office with a dedicated printer, by email, or by automated faxing. These computers can track test orders, provide pre-analytical information, assist in quality control and quality assurance procedures, alert laboratory staff of an unusual finding such as a critical value, and report and store all laboratory results. Laboratory reports generated by the system can also highlight values that fall outside the expected or reference interval to help the provider focus on the tests that are of most concern.

Here is one example of a laboratory report containing the elements described above.

A healthcare provider is expected to evaluate all of the relevant findings – laboratory test data plus information from other sources, such as physical exam, personal and family histories, signs and symptoms, and other diagnostic examinations, i.e., X-rays, EKG, etc. – before settling on a diagnosis and developing a treatment plan. Given the complexities in human physiology and disease response, no diagnosis should ever be made solely on the basis of a single lab test. The clinician must always ask, “Do the test data fit with the other pieces of the puzzle?” Careful evaluation and consideration of test findings increase the reliability of a diagnosis and can reduce the chance of medical errors.

Data from medical tests are part of the information set that needs to be considered when a healthcare provider makes a diagnosis. When a laboratory report indicates abnormal or unexpected results, it is incumbent on your healthcare provider to further evaluate and corroborate the information at hand to ensure an accurate diagnosis. If the data do not correspond with the clinical picture, additional information may be needed and retesting may be appropriate. In some situations, the progression of the disease or condition may not be evident for the testing modality to be relevant.

Is there anything I can do?

Yes. Sometimes there are things you do or don’t do that can affect your tests results. Following a couple of guidelines can help to ensure that your results are interpreted correctly by your healthcare provider:

• Your healthcare provider, medical office staff, the sample collection site, or the laboratory performing the testing should discuss with you how to prepare for a test to avoid known interferences. You may be instructed to fast or avoid certain foods or activities. Carefully follow these instructions to prepare for the test.
• Since some test results can be affected by medications, vitamins, and other over-the-counter health supplements, it is important that you provide a complete and honest medical history so that your provider can correctly interpret the results from the lab.

If you and your provider are surprised by a test result, here are some questions you could discuss together.

1. Is the result consistent with my symptoms and current health state?
2. How often do false-negative or false-positive results occur with this test?
3. Is the course of action being considered serious enough that we should retest?
4. Do we have any reason to doubt the quality of the findings?
5. Would it be appropriate in this case to get a second opinion?

Laboratory testing is a science professionally conducted with rigorous statistical analysis, quality controls, and extensive oversight. Medical testing is an important component in the diagnostic tool kit of a healthcare provider. However, it is most reliable when used in conjunction with other meaningful data collected in the diagnostic process, when appropriate questions are asked and answered, and when there is open communication between healthcare teams and patients.

You can put great trust in the data generated by today’s exacting medical tests. A caveat to remember is that a diagnosis and treatment plan for a serious disease should never be based on a single medical test. Test findings are just one of the pieces of the diagnostic puzzle.

Help your healthcare provider with the interpretation of the data by providing thorough information on your medical history and keeping track of symptoms you wish to report. To reduce the chance that you are improperly diagnosed and given potentially harmful and unnecessary medical procedures, first follow test preparation instructions carefully and tell your healthcare provider and the person collecting your sample about any exceptions you made.

Accessing your lab report after you have testing done is a good way to stay informed and you can follow up with your healthcare provider if you have any questions concerning your results.